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It's no secret that marijuana can put a smile on many people's faces, but research suggests that the drug's positive effects go beyond just getting high. A 2012 study published in the peer-reviewed academic journal European Neuropsychopharmacology suggests that the brain's endocannabinoid system – which is activated by THC, the psychoactive ingredient in marijuana – may play an important role in emotional processing, "an essential aspect of appropriate social interactions and interpersonal relationships."

Specifically, the study's authors found that participants given THC in a controlled experiment showed lower brain activity in response negative stimuli than did those given placebo. A bias toward negative stimuli has been linked to mental illnesses like depression, and evidence that THC reduces this effect suggests that the endocannabinoid system could play an important, beneficial role in how humans experience emotions and mood.

Researchers measured test-specific effects of THC administration on about a dozen men who had used marijuana at least four times in the past year, but no more than once a week. Half of them were given THC, the other half placebo; the researchers then showed all the men images of faces with expressions that appeared either "fearful" or "happy." They found that participants given THC showed significantly decreased accuracy in matching facial expressions with negative emotion, but showed about the same accuracy for positive associations. Using brain imaging technology called fMRI, they were also able to watch the effects of THC on the parts of the participants' brains that process emotion – identifying a "network-wide shift from a bias for negative emotional content towards a bias for positive emotional content."

The researchers concluded that the way the human brain reacts to THC could have significant implications for mental health treatment. "These findings," they wrote, "add to existing evidence that implicate the endocannabinoid system in modulation of emotional reactions, and support a previously suggested role for the endocannabinoid system in abnormal emotional processing associated with various psychiatric disorders."